The present invention relates to an operating apparatus for operating a discharge lamp (discharge lamp operating apparatus) and a self-ballasted electrodeless discharge lamp.
In recent years, as discharge lamp operating apparatuses, electronically operating apparatuses (inverter) are widely used in view of improvement of luminous efficiency, and compactness and lightness of an apparatus. Examples of such a discharge lamp operating apparatus are disclosed in Japanese Laid-Open Patent Publication Nos. 10-162983 and 2000-353598. The conventional discharge lamp operating apparatuses will be described below.
FIG. 6 shows the structure of the discharge lamp operating apparatus disclosed in Japanese Laid-Open Patent Publication No. 2000-353598. The discharge lamp operating apparatus shown in FIG. 6 includes a DC power 22, an inverter circuit 31, a control terminal driving circuit 23, and a resonant load circuit 30 including a fluorescent lamp 8.
The DC power 22 includes an AC power 1, a noise prevention capacitor 2, a diode bridge 3 and a smoothing capacitor 4, and the structure of the DC power 22 is a common structure in which the alternating current of the AC power 1 is converted to a direct current and output the direct current. The inverter circuit 31 includes FETs 5 and 6, which are switching elements with control terminals (gate). In the inverter circuit 31, the FETs 5 and 6 are turned on or off alternately, so that the direct current is converted to an alternating current. The FETs 5 and 6 are controlled to be on or off by the control terminal driving circuit 23, and the control terminal driving circuit 23 includes a secondary winding 10A of a transformer 10, which is a drive winding, and a capacitor 14, a second capacitor 17, Zener diodes 15 and 16, which are voltage clamp elements, resistors 12, 13, and 19, and a second inductor 11.
The fluorescent lamp 8, which is a discharge lamp, is a part of the resonant load circuit 30, and the resonant circuit 30 includes the fluorescent lamp 8, a first capacitor 7, a capacitor 9 for preheating electrodes 8A and 8B of the fluorescent lamp 8, a primary winding 10B of the transformer 10, which is a first inductor.
In the structure as described above, a direct current output from the DC power 22 is converted to be an alternating current by the control terminal driving circuit 23 turning the FETs 5 and 6 on or off alternately, and the alternating current is applied to the resonant load circuit 30, so that the fluorescent lamp 8 can be operated with the alternating current.
Next, the operation of the discharge lamp operating apparatus having the structure shown in FIG. 6 will be described.
When the AC power 1 is turned on, a pulsating current waveform that has been full-wave rectified by the rectifier 3 is smoothed by the capacitor 4, and a DC voltage that substantially corresponds to the peak value of the AC power 1 is generated at both ends of the capacitor 4.
The DC voltage generated at both ends of the capacitor 4 is applied to a series circuit of the FETs 5 and 6, which is the inverter circuit 31, and also applied to a series circuit of the resonant load circuit 30 and the resistor 19 so as to charge the capacitors 7 and 9.
At the same time, the DC voltage generated at both ends of the capacitor 4 is applied to a series circuit of the resistor 12, the inductor 11, the secondary winding 10A of the transformer 10, the capacitor 14, and the resistor 19 in the control terminal driving circuit 23, and the capacitor 14 is charged with charges at a predetermined time constant. The maximum voltage that can be generated at both ends of the capacitor 14 is not more than a voltage that is generated at both ends of the resistor 13 when the DC voltage generated at both ends of the capacitor 4 is divided between the resistors 12, 13, and 19.
In this case, when the voltage of the capacitor 14 charged at a predetermined time constant reaches the Zener voltage of the Zener diode 15, the charges of the capacitor 14 are supplied to the gate terminal of the FET 5 so that the FET 5 is turned on. When the FET 5 is turned on, the charges charged to the capacitors 7 and 9 are discharged via the FET 5 and the primary winding 10B of the transformer 10. Here, the current flowing through the primary winding 10B of the transformer 10 generates an induced voltage in the secondary winding 10A of the transformer 10, and the induced voltage in the secondary winding 10A of the transformer 10 allows the series resonant circuit constituted by the inductor 11 and the capacitor 17 to oscillate at a resonance frequency determined by the inductor 11 and the capacitor 17, so that an oscillation voltage is generated at both ends of the capacitor 17.
This oscillation voltage keeps the FET 5 on for a predetermined period, and thereafter a voltage in the reverse bias direction is generated between the gate and the source of the FET 5, so that the FET 5 is turned off. At the same time, a voltage in the forward bias direction is applied between the gate and the source of the FET 6, so that the FET 6 is turned on. When the FET 6 is turned on, using the DC voltage generated at both ends of the capacitor 4 as the power source, a current flows through the capacitor 7, the fluorescent lamp 8, the primary winding 10B of the transformer 10, and the FET 6. This current allows the capacitors 7 and 9 to be charged.
In this case, the current flowing through the primary winding 10B of the transformer 10 flows in a direction opposite to the direction when the FET 5 is on, and an induced voltage having a polarity opposite to that in the above case is generated in the secondary winding 10A of the transformer 10, and an oscillation operation is performed at a resonance frequency determined by the inductor 11 and the capacitor 17 and an oscillation voltage is generated at both ends of the capacitor 17. This oscillation voltage keeps the FET 6 on for a predetermined period, and thereafter a voltage in the reverse bias direction is generated between the gate and the source of the FET 6, so that the FET 6 is turned off. At the same time, a voltage in the forward bias direction is applied between the gate and the source of the FET 5, so that the FET 5 is turned on.
Thereafter, the FETs 5 and 6 repeat to be on or off alternately, so that an alternating current is applied to the resonant load circuit 30. When the alternating current is applied to the resonant load circuit 30 and immediately after the AC power 1 is introduced, the current flows through the capacitor 7, the electrode 8A, the capacitor 9, the electrode 8B, and the primary winding 10B of the transformer 10 so that a preheating current is supplied to the electrodes 8A and 8B of the fluorescent lamp 8. Since the resonant load circuit 30 constitutes a series resonant circuit, a high voltage as a resonance voltage is generated at both ends of the capacitor 9 when the preheating current is supplied.
Then, the temperature of the electrodes is increased by the preheating current to the electrodes 8A and 8B, so that thermoelectrons are easily generated and the high voltage at both the ends of the capacitor 9 is applied to both ends of the fluorescent lamp 8. Therefore, the fluorescent lamp 8 starts discharge. When the fluorescent lamp 8 starts discharge, the impedance of the fluorescent lamp 8 is decreased, and most of the current flowing through the capacitor 7 flows through the fluorescent lamp 8 so that stable discharge can be maintained.
According to the conventional structure, the fluorescent lamp 8 can be operated without particular problems, but when the inventors of the present invention made experiments regarding the operation when the fluorescent lamp 8 having electrodes is replaced by an electrodeless fluorescent lamp, it was found that the electrodeless fluorescent lamp with this structure was not operated reliably. Furthermore, it was also found that in the case of the fluorescent lamp 8 having electrodes, this structure was not sufficient to operate the fluorescent lamp 8 reliably in any environment.
On the other hand, adding significant changes to the structure of the operating apparatus for operating the fluorescent lamp 8 results in an increase of the cost of products, and it is also necessary to examine the structure with the added changes carefully whether or not any new problem may be caused.